ABSTRACT
We present the cloning and characterization of two novel calcium-activated potassium channel beta subunits, hKCNMB3 and hKCNMB4, that are enriched in the testis and brain, respectively. We compare and contrast the steady state and kinetic properties of these beta subunits with the previously cloned mouse beta1 (mKCNMB1) and the human beta2 subunit (hKCNMB2). Once inactivation is removed, we find that hKCNMB2 has properties similar to mKCNMB1. hKCNMB2 slows Hslo1 channel gating and shifts the current-voltage relationship to more negative potentials. hKCNMB3 and hKCNMB4 have distinct effects on slo currents not observed with mKCNMB1 and hKCNMB2. Although we found that hKCNMB3 does interact with Hslo channels, its effects on Hslo1 channel properties were slight, increasing Hslo1 activation rates. In contrast, hKCNMB4 slows Hslo1 gating kinetics, and modulates the apparent calcium sensitivity of Hslo1. We found that the different effects of the beta subunits on some Hslo1 channel properties are calcium-dependent. mKCNMB1 and hKCNMB2 slow activation at 1 microM but not at 10 microM free calcium concentrations. hKCNMB4 decreases Hslo1 channel openings at low calcium concentrations but increases channel openings at high calcium concentrations. These results suggest that beta subunits in diverse tissue types fine-tune slo channel properties to the needs of a particular cell.
Subject(s)
Potassium Channels, Calcium-Activated , Potassium Channels/genetics , Amino Acid Sequence , Brain/metabolism , Calcium/pharmacology , Cloning, Molecular , Humans , Ion Channel Gating , Kinetics , Large-Conductance Calcium-Activated Potassium Channel alpha Subunits , Large-Conductance Calcium-Activated Potassium Channel beta Subunits , Large-Conductance Calcium-Activated Potassium Channels , Male , Molecular Sequence Data , Nerve Tissue Proteins/chemistry , Patch-Clamp Techniques , Potassium Channels/chemistry , RNA, Messenger/metabolism , Sequence Alignment , Testis/metabolismABSTRACT
The aim of the present study was to assess differences in transient outward potassium current (Ito) between the right ventricular free wall and the interventricular septum of the adult rat ventricle and to evaluate the relative contributions of Kv4.2, Kv4.3, and Kv1.4 to Ito in these regions. The results show that Ito is composed of both rapidly and slowly recovering components in the right wall and septum. The fast component had a significantly higher density in the right free wall than in the septum, whereas the slow component did not differ between the two sites. Kv4.2 mRNA and protein levels were also highest in the right wall and correlated with Ito density, whereas Kv4.3 was expressed uniformly in these regions. The kinetics of the rapidly recovering component of Ito in myocytes was similar to that recorded in tsa-201 cells expressing Kv4.2 and Kv4.3 channels. Kv1.4 mRNA and protein expression correlated well with the density of the slowly recovering Ito, whereas the recovery kinetics of the slow component were identical to Kv1.4 expressed in tsa-201 cells. In conclusion, expression of Kv1.4, Kv4.2, and Kv4.3 differs between regions in rat hearts. Regionally specific differences in the genetic composition of Ito can account for the region-specific properties of this current.
